If the latest an activity can start is the earliest it must start, it is critical. Critical tasks show the same early and late dates on the forward pass as on the backward pass and in normally have no float.

A task requiring human performance which, if not accomplished in accordance with system requirements, will most likely have adverse effects on cost, system reliability, efficiency, effectiveness, or safety. A task is also considered critical whenever equipment design characteristics demand human performance which approaches the limits of human capabilities.